Pynamodb

Backporting 1083 update for a botocore 1.28 private API change which caused the following exception:

TypeError: _convert_to_request_dict() missing 1 required positional argument: 'endpoint_url'


**Full Changelog**: https://github.com/pynamodb/PynamoDB/compare/5.2.1...5.2.2

This is a backwards compatible, minor release.

Please read the [release notes](https://github.com/pynamodb/PynamoDB/blob/master/docs/release_notes.rst#v521) for a full list of changes

This is a backwards compatible, minor release.

Please read the [release notes](https://github.com/pynamodb/PynamoDB/blob/master/docs/release_notes.rst#v520) for a full list of changes.

* Introduce ``DynamicMapAttribute`` to enable partially defining attributes on a ``MapAttribute`` (868)
* Quality of life improvements: Type annotations, better comment, more resilient test (934, 936, 948)
* Fix type annotation of ``is_in`` conditional expression (947)
* Null errors should include full attribute path (915)
* Fix for serializing and deserializing dates prior to year 1000 (949)

It is more efficient not to serialize attributes you don't need, and it also avoids tripping null-checks on unrelated attributes when doing updates or deletes.